They should do 1 more and end it all in a very good way using some of the older cast from the 70's. I like the idea of an original 70's murderer making a comeback. Also Columbo likes the nightlife would be an unsatisfactory way to end it all - really push the boat out.
Hopefully the reality faze is over as i didnt like that idea at all.
If they were to do only one more episode then it should have a finality to it. I think when the episode ends Columbo should retire, whether we know it from the beginng (wouldn't that be ironic) or it is a surprise to us all at the end.